Bear Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bear Observatory clear sky chart from 2005-11-06 to 2026-07-16.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	13% 8% 3 7% 5% 4 5% 4 6% 7% 39%	137723
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 40%	135067
Transparency	Day	43% 4 11% 24% 16% 2	141377
	Night	44% 4 10% 22% 18% 3	139487
Seeing	Day	17% 11% 24% 24% 17% 8%	17585
	Night	18% 8% 25% 30% 17% 2	27289

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 11% 7% 11% 5% 4 4 3 5% 5% 30%	4691
	11th to 20th	Day	12% 9% 4 11% 5% 4 4 2 3 4 42%	4784
	21st to end	Day	21% 10% 6% 6% 4 4 4 3 4 5% 33%	6056
February	1st to 10th	Day	18% 12% 4 7% 4 4 3 3 5% 6% 35%	2100
	11th to 20th	Day	20% 9% 4 8% 5% 4 4 3 5% 6% 33%	2048
	21st to end	Day	17% 10% 4 8% 5% 4 4 4 5% 5% 34%	1730
March	1st to 10th	Day	19% 12% 4 7% 4 4 4 3 3 5% 36%	4580
	11th to 20th	Day	13% 10% 2 6% 4 3 4 3 4 6% 45%	4935
	21st to end	Day	12% 8% 3 8% 5% 5% 4 3 5% 6% 41%	5809
April	1st to 10th	Day	13% 6% 2 6% 5% 4 4 4 6% 8% 41%	2520
	11th to 20th	Day	14% 7% 3 7% 6% 4 4 4 4 6% 42%	2494
	21st to end	Day	15% 7% 3 6% 4 5% 4 3 6% 8% 38%	2639
May	1st to 10th	Day	17% 8% 3 8% 4 5% 5% 4 6% 8% 31%	7616
	11th to 20th	Day	15% 7% 3 7% 4 4 4 4 6% 7% 38%	7626
	21st to end	Day	18% 8% 3 8% 5% 5% 5% 4 6% 8% 29%	8391
June	1st to 10th	Day	10% 6% 3 6% 5% 5% 5% 4 6% 7% 42%	2802
	11th to 20th	Day	6% 5% 2 5% 5% 5% 6% 4 7% 10% 45%	2940
	21st to end	Day	7% 5% 2 5% 4 5% 6% 4 9% 10% 43%	2940
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 5% 5% 6% 10% 40%	6500
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 11% 36%	6390
	21st to end	Day	10% 9% 3 9% 6% 6% 6% 5% 8% 11% 28%	6424
August	1st to 10th	Day	5% 4 2 5% 4 4 4 5% 7% 11% 50%	4215
	11th to 20th	Day	8% 5% 2 6% 4 5% 6% 5% 8% 10% 41%	3849
	21st to end	Day	5% 3 2 5% 4 3 5% 4 5% 9% 55%	4312
September	1st to 10th	Day	12% 6% 3 7% 4 5% 5% 4 5% 7% 42%	2351
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 6% 8% 46%	2200
	21st to end	Day	14% 5% 2 6% 3 4 4 3 5% 7% 48%	2200
October	1st to 10th	Day	10% 5% 2 5% 3 4 3 2 4 6% 57%	2200
	11th to 20th	Day	12% 6% 2 4 4 3 4 3 4 6% 52%	2126
	21st to end	Day	18% 8% 2 6% 3 3 3 3 4 5% 44%	1980
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 4 6% 53%	1852
	11th to 20th	Day	16% 8% 2 5% 4 3 3 3 5% 6% 45%	1890
	21st to end	Day	17% 9% 3 6% 4 4 4 3 4 6% 41%	1890
December	1st to 10th	Day	17% 10% 2 7% 4 4 3 2 4 5% 41%	3020
	11th to 20th	Day	17% 11% 5% 10% 5% 5% 4 3 4 5% 31%	3463
	21st to end	Day	20% 15% 4 8% 6% 5% 4 3 4 6% 25%	4160
January	1st to 10th	Night	15% 14% 5% 9% 4 3 4 3 5% 6% 33%	8232
	11th to 20th	Night	15% 11% 4 9% 5% 5% 4 3 4 4 36%	8184
	21st to end	Night	18% 11% 4 7% 4 3 3 2 3 5% 38%	8120
February	1st to 10th	Night	21% 11% 3 8% 4 4 4 3 5% 6% 30%	2520
	11th to 20th	Night	21% 9% 4 7% 5% 4 4 3 3 5% 35%	2458
	21st to end	Night	21% 10% 3 8% 4 4 3 3 4 5% 34%	2076
March	1st to 10th	Night	19% 14% 3 7% 4 4 3 3 4 4 34%	5493
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 5% 6% 48%	5243
	21st to end	Night	13% 8% 3 9% 5% 5% 5% 3 5% 6% 38%	5405
April	1st to 10th	Night	12% 6% 2 5% 5% 3 5% 4 6% 7% 44%	2100
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 6% 7% 42%	2075
	21st to end	Night	14% 8% 2 6% 5% 4 4 3 6% 8% 38%	1979
May	1st to 10th	Night	16% 10% 3 7% 5% 5% 5% 4 6% 8% 31%	5237
	11th to 20th	Night	12% 7% 3 9% 5% 6% 5% 4 6% 8% 36%	5234
	21st to end	Night	16% 8% 3 8% 5% 5% 5% 4 6% 9% 31%	5761
June	1st to 10th	Night	9% 6% 1 5% 4 3 4 5% 6% 9% 46%	1812
	11th to 20th	Night	6% 4 2 4 5% 4 5% 3 7% 10% 50%	1680
	21st to end	Night	4 4 2 4 5% 4 5% 5% 8% 12% 47%	1680
July	1st to 10th	Night	7% 4 2 6% 4 5% 5% 4 6% 9% 47%	3698
	11th to 20th	Night	7% 4 2 5% 4 5% 5% 4 7% 12% 45%	3691
	21st to end	Night	8% 7% 3 10% 7% 6% 6% 5% 8% 10% 32%	4427
August	1st to 10th	Night	6% 4 2 5% 3 3 3 3 5% 8% 58%	2930
	11th to 20th	Night	5% 4 2 6% 4 4 5% 4 7% 10% 49%	3200
	21st to end	Night	4 3 1 3 3 3 3 3 6% 8% 64%	3586
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 5% 6% 50%	2027
	11th to 20th	Night	10% 5% 2 5% 4 4 3 3 5% 9% 50%	2200
	21st to end	Night	12% 4 2 5% 3 4 4 4 4 6% 52%	2200
October	1st to 10th	Night	9% 4 2 4 2 3 3 2 4 6% 61%	2200
	11th to 20th	Night	14% 5% 2 6% 3 3 3 3 4 5% 53%	2268
	21st to end	Night	17% 7% 2 5% 3 4 4 3 3 6% 47%	2860
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 3 4 4 51%	2666
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 4 6% 47%	2730
	21st to end	Night	17% 9% 3 5% 3 3 3 3 4 5% 42%	2730
December	1st to 10th	Night	17% 12% 3 7% 4 3 3 3 4 4 39%	4380
	11th to 20th	Night	21% 9% 4 7% 3 3 3 3 4 6% 36%	4952
	21st to end	Night	21% 16% 5% 10% 6% 5% 4 3 3 4 23%	7033

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	57% 2 11% 21% 8% 1	4692
	11th to 20th	Day	48% 2 5% 25% 19% 2	4784
	21st to end	Day	54% 2 8% 21% 11% 4	6056
February	1st to 10th	Day	52% 4 9% 22% 13% 2	2100
	11th to 20th	Day	53% 4 8% 21% 12% 3	2058
	21st to end	Day	51% 4 8% 18% 14% 4	1730
March	1st to 10th	Day	52% 2 5% 17% 19% 5%	4580
	11th to 20th	Day	41% 3 8% 25% 19% 3	4935
	21st to end	Day	44% 4 10% 28% 13% 1	5811
April	1st to 10th	Day	40% 6% 12% 23% 17% 2	2520
	11th to 20th	Day	44% 4 10% 22% 17% 3	2494
	21st to end	Day	44% 6% 10% 26% 13% 2	2639
May	1st to 10th	Day	50% 5% 12% 22% 11% 1	7651
	11th to 20th	Day	45% 4 12% 23% 15%	7630
	21st to end	Day	52% 5% 11% 17% 13% 2	8405
June	1st to 10th	Day	39% 7% 11% 22% 19% 2	2802
	11th to 20th	Day	32% 8% 15% 25% 18% 2	2940
	21st to end	Day	33% 8% 16% 21% 18% 4	2940
July	1st to 10th	Day	40% 5% 13% 25% 16% 1	6431
	11th to 20th	Day	40% 6% 16% 28% 9% 2	6315
	21st to end	Day	49% 5% 18% 20% 7% 1	6240
August	1st to 10th	Day	27% 6% 14% 35% 16% 1	4202
	11th to 20th	Day	37% 5% 13% 27% 16% 1	3885
	21st to end	Day	26% 5% 10% 39% 17% 3	4311
September	1st to 10th	Day	39% 7% 12% 20% 18% 4	2351
	11th to 20th	Day	36% 6% 9% 24% 23% 3	2200
	21st to end	Day	36% 5% 8% 25% 22% 4	2200
October	1st to 10th	Day	21% 3 5% 22% 46% 4	3590
	11th to 20th	Day	24% 3 9% 27% 32% 5%	3433
	21st to end	Day	32% 3 6% 28% 23% 8%	3180
November	1st to 10th	Day	34% 4 8% 28% 22% 4	1852
	11th to 20th	Day	41% 3 8% 23% 20% 5%	1890
	21st to end	Day	46% 3 8% 23% 18% 2	1890
December	1st to 10th	Day	47% 2 7% 25% 18% 1	3020
	11th to 20th	Day	57% 2 8% 17% 13% 2	3460
	21st to end	Day	61% 4 11% 18% 7%	4160
January	1st to 10th	Night	53% 3 12% 23% 8% 1	8233
	11th to 20th	Night	53% 2 4 20% 17% 4	8184
	21st to end	Night	52% 2 7% 20% 18% 2	8120
February	1st to 10th	Night	56% 4 9% 18% 12% 1	2520
	11th to 20th	Night	53% 4 6% 21% 13% 2	2459
	21st to end	Night	54% 4 8% 18% 14% 3	2076
March	1st to 10th	Night	54% 3 5% 18% 16% 4	5493
	11th to 20th	Night	38% 3 8% 25% 20% 6%	5243
	21st to end	Night	47% 3 10% 24% 15% 1	5405
April	1st to 10th	Night	38% 7% 12% 23% 19% 3	2100
	11th to 20th	Night	41% 5% 10% 18% 21% 5%	2075
	21st to end	Night	43% 6% 9% 24% 16% 2	1979
May	1st to 10th	Night	52% 5% 11% 22% 10% 1	5264
	11th to 20th	Night	46% 6% 12% 20% 15%	5235
	21st to end	Night	49% 6% 13% 20% 11% 1	5771
June	1st to 10th	Night	33% 6% 13% 26% 18% 3	1812
	11th to 20th	Night	28% 6% 14% 25% 24% 3	1680
	21st to end	Night	27% 10% 17% 22% 19% 5%	1680
July	1st to 10th	Night	34% 6% 13% 28% 19% 2	3660
	11th to 20th	Night	33% 7% 17% 29% 11% 2	3646
	21st to end	Night	46% 6% 16% 21% 9% 1	4297
August	1st to 10th	Night	25% 4 12% 40% 18% 1	2919
	11th to 20th	Night	30% 5% 16% 29% 18% 1	3230
	21st to end	Night	20% 5% 11% 37% 24% 4	3586
September	1st to 10th	Night	34% 7% 9% 21% 24% 5%	2027
	11th to 20th	Night	32% 4 10% 26% 24% 4	2200
	21st to end	Night	33% 5% 8% 23% 25% 6%	2200
October	1st to 10th	Night	20% 3 5% 18% 49% 6%	3590
	11th to 20th	Night	23% 2 6% 24% 37% 8%	3700
	21st to end	Night	32% 2 7% 21% 31% 8%	4610
November	1st to 10th	Night	36% 4 7% 24% 23% 5%	2666
	11th to 20th	Night	38% 4 8% 22% 20% 7%	2730
	21st to end	Night	45% 4 8% 23% 18% 3	2730
December	1st to 10th	Night	50% 3 8% 21% 17% 1	4380
	11th to 20th	Night	51% 2 9% 20% 16% 2	4954
	21st to end	Night	66% 3 9% 16% 6% 1	7033

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	26% 19% 30% 18% 6% 1	330
	11th to 20th	Day	28% 28% 28% 12% 4	330
	21st to end	Day	23% 23% 39% 12% 3	363
February	1st to 10th	Day	23% 20% 40% 13% 3	330
	11th to 20th	Day	32% 22% 34% 9% 3	330
	21st to end	Day	29% 21% 36% 12% 3	273
March	1st to 10th	Day	25% 22% 36% 13% 3 1	330
	11th to 20th	Day	25% 19% 37% 13% 6% 1	463
	21st to end	Day	24% 20% 35% 15% 6% 1	594
April	1st to 10th	Day	21% 17% 28% 25% 9% 1	540
	11th to 20th	Day	18% 20% 27% 22% 11% 2	532
	21st to end	Day	19% 15% 32% 22% 9% 3	540
May	1st to 10th	Day	19% 14% 31% 24% 7% 4	543
	11th to 20th	Day	17% 9% 31% 25% 12% 6%	567
	21st to end	Day	15% 7% 21% 29% 18% 10%	617
June	1st to 10th	Day	11% 5% 24% 25% 24% 11%	642
	11th to 20th	Day	8% 4 20% 33% 25% 10%	780
	21st to end	Day	9% 3 21% 29% 24% 13%	780
July	1st to 10th	Day	10% 3 14% 26% 28% 19%	771
	11th to 20th	Day	9% 4 14% 30% 28% 15%	729
	21st to end	Day	11% 3 11% 29% 28% 18%	576
August	1st to 10th	Day	12% 1 13% 26% 30% 18%	526
	11th to 20th	Day	11% 4 13% 32% 29% 11%	519
	21st to end	Day	10% 3 10% 25% 32% 21%	583
September	1st to 10th	Day	13% 3 14% 23% 33% 14%	530
	11th to 20th	Day	12% 2 13% 25% 34% 16%	530
	21st to end	Day	17% 6% 19% 26% 22% 10%	530
October	1st to 10th	Day	12% 4 18% 32% 27% 8%	530
	11th to 20th	Day	14% 12% 29% 25% 14% 7%	501
	21st to end	Day	24% 14% 29% 24% 9% 1	363
November	1st to 10th	Day	18% 11% 32% 27% 9% 3	330
	11th to 20th	Day	22% 15% 34% 22% 7%	330
	21st to end	Day	27% 13% 32% 20% 8% 1	330
December	1st to 10th	Day	22% 19% 33% 18% 7% 1	330
	11th to 20th	Day	22% 18% 33% 22% 5%	330
	21st to end	Day	31% 17% 32% 16% 4	363
January	1st to 10th	Night	23% 15% 31% 27% 4	1050
	11th to 20th	Night	27% 16% 32% 21% 4	1019
	21st to end	Night	23% 14% 39% 23% 1	924
February	1st to 10th	Night	26% 12% 36% 23% 3	840
	11th to 20th	Night	26% 15% 36% 21% 2	823
	21st to end	Night	28% 14% 38% 20% 1	692
March	1st to 10th	Night	22% 15% 33% 26% 5%	840
	11th to 20th	Night	22% 14% 34% 24% 5% 1	840
	21st to end	Night	20% 12% 33% 30% 5%	844
April	1st to 10th	Night	16% 13% 33% 29% 8% 1	630
	11th to 20th	Night	20% 11% 30% 27% 12% 1	623
	21st to end	Night	18% 8% 31% 32% 10% 1	630
May	1st to 10th	Night	21% 6% 27% 32% 12% 2	630
	11th to 20th	Night	14% 4 21% 39% 18% 2	621
	21st to end	Night	9% 3 18% 38% 27% 5%	683
June	1st to 10th	Night	12% 1 18% 36% 30% 3	630
	11th to 20th	Night	7% 2 14% 41% 31% 4	630
	21st to end	Night	5% 2 16% 39% 33% 5%	630
July	1st to 10th	Night	11% 1 8% 28% 44% 7%	615
	11th to 20th	Night	10% 2 6% 33% 43% 6%	597
	21st to end	Night	10% 2 10% 35% 36% 7%	657
August	1st to 10th	Night	9% 2 8% 31% 44% 6%	597
	11th to 20th	Night	7% 1 10% 37% 39% 6%	594
	21st to end	Night	7% 1 7% 33% 43% 8%	660
September	1st to 10th	Night	11% 2 12% 33% 37% 5%	627
	11th to 20th	Night	11% 2 11% 32% 40% 5%	800
	21st to end	Night	15% 3 18% 32% 29% 4	800
October	1st to 10th	Night	12% 3 17% 33% 30% 5%	800
	11th to 20th	Night	17% 7% 23% 34% 18% 3	800
	21st to end	Night	21% 7% 31% 27% 13% 1	880
November	1st to 10th	Night	14% 8% 29% 32% 15% 1	820
	11th to 20th	Night	17% 11% 31% 30% 10%	840
	21st to end	Night	23% 13% 24% 28% 11%	840
December	1st to 10th	Night	27% 11% 29% 29% 5%	840
	11th to 20th	Night	26% 9% 34% 26% 6%	840
	21st to end	Night	29% 13% 30% 26% 3	1103

Page generated at 2026-07-13 02:14:47. Version ac07

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